Friday, 7 July 2017

Visits from a leaf cutter

Our friends Fiona and Conor gave us a potted houseleek plant (Sempervivum sp.) and it has grown so
well that it has now been divided. Four of the rosettes were
re-planted into the attractive ceramic pot that was part of the original gift
and this forms the central decoration on the large table in one of the seating areas
in our garden.

Yesterday, while enjoying a cup of coffee in the warm sunshine, I noticed
that the pot was being visited by a leafcutter bee (Megachile sp.). It flew around the pot, landed on the soil at the
margin, and then disappeared under the lip. Although my photographic skills are
less well developed than my patience, I was able to get an image of the bee in
flight just before it landed (see above). In addition to making frequent
foraging flights for pollen and nectar, it also brought in sections of leaf on
three successive occasions and the bee then looked just like the one in this wonderful
image taken by HymenOphrysand published
on the urbanbees website [1]:

To find out more about the bee’s behaviour, I looked up Shuckard's British Bees [2]. The prose of
Nineteenth Century Natural Historians is often appealing, as they had few other
methods of describing their observations, only being let down by excesses of
anthropomorphism. Shuckard was no exception, and he conveys his passion for the
study of insects, something which overtook his ability to manage his finances
[3]. These are his descriptions of the behaviour of this solitary bee [2]:

The proceedings of these bees are very curious.
Although the tubes they usually form are long, they are so constructed as not
to branch far away from the exterior of the material into which they bore..
..Both the sides of the tube, and the cells they form within them, will
necessarily vary in diameter and length with the size of the species, but in
the larger species they are about an inch and a quarter long and half an inch
in diameter..

..The cylindrical tube [is] prepared.. ..by the
gradual removal of the particles of the wood, or sand, or earth of which it
consists, the insect's instinct [then] prompts it to fly forth to obtain the
requisite lining, that the lateral earth may not fall in, or the wood taint the
store to be accumulated for the young.. .. Having fixed upon the preferred
plant .. ..it alights upon the leaf, and fixing itself upon the edge, it holds
it with three legs on each side, then using its mandibles as the cutter of
silhouettes would his scissors, and, just as rapidly as he cuts out a profile,
does this ingenious little creature ply the tools it is furnished with by
nature. The oval or semicircular cutting being thus speedily dispatched, with
the legs still clinging to the surfaces, the insect biting its way backwards,
the piece cut off necessarily remains within the clutch of the legs, and, when
about falling, the rejoicing labourer expands her wings and flies off.. ..[to
arrive] at the mouth of the aperture within which she has to convey it..

Shuckhard then describes how other pieces of leaf are brought in to
complete the lining and it was this that I was watching. The bee then collects
pollen and nectar (flights that I also saw), and deposits these in the
leaf-lined tube. An egg is laid on to the mass and the cell completed by another
leaf fragment of circular form.

Shuckard continues:

The whole process is again renewed in the same manner
as at first, the bottom edge of the cutting of the external leaf is again
curved to form a concave bottom to the next cell, and the sides are similarly
formed, and each cell fits the preceding like the top of one thimble placed in
the mouth of another.

The analogy to thimbles is rather dated, but we can now picture more clearly
the biology of the bee. In time, larvae
hatch from the eggs, consume the food store provided for them, and grow through
stages to then produce a silk cocoon in which they pupate. Emergence of the adults from
pupae completes the cycle, with mating occurring and the next generation
of female bees starting to build tubes and cut leaves.

It is intriguing to ask oneself how the habits of the bee evolved: its
burrowing behaviour; the use of leaves to provide a lining of the tubes it makes; the
manner in which leaf segments are brought together; the collection of pollen
and nectar; the evolution of life stages of such markedly different form; and
the ability to fly and to navigate. All this was going through my mind when
watching "my" bee and I'm so pleased that she chose our pot of Sempervivum as the home for the next
generation. They will not be disturbed.